U.S. Pat. No. 2,939,952 to Paul et al describes the quadrupole mass filter. This device consists of four parallel hyperbolic electrically conducting (metal) sheets or circular rods to which a combination of radio-frequency (rf) and direct-current (dc) voltages are applied. If the values of the dc voltages and the amplitudes and frequencies of the rf voltages are selected correctly, only the ions of a specific mass-to-charge ratio are transmitted from one end of the quadrupole structure to the other. Ions with mass-to-charge ratios other than the ratio desired for transmission are on unstable trajectories and are rejected by moving transversely to the axis so they strike the poles and are electrically neutralized. The Paul et al patent also teaches that if only rf fields are applied to the structure, then ions of all mass-to-charge ratios in excess of a given value determined by the amplitude and frequency of the rf voltage applied will be transmitted, and those with lower mass-to-charge ratios will be on unstable trajectories and will be rejected.
U.S. Pat. No. 3,129,327 to W. M. Brubaker teaches that an ion which is or would be on a stable trajectory within a quadrupole mass filter structure must pass through fringe fields from and adjacent to the ends of the structure, to enter or to leave the structure, and that while in the fringe fields, the selected ions are on unstable trajectories and can be rejected before reaching the quadrupole structure. The patent further teaches that a way to avoid rejection in the fringe fields at the entrance of the quadrupole structure is to place immediately before the quadrupole structure to which both rf and dc voltages are applied, a very short quadrupole structure to which only rf voltages are applied. In this way an entering ion "sees" first the rf fields which become substantial before it "sees" the dc fringe fields. This has been called by Brubaker the "delayed dc ramp". This arrangement keeps the entering selected ions on stable trajectories at all times while in the fringe fields and results in improved overall transmission of ions by the device and therefore improved sensitivity and performance.
U.S. Pat. Nos. 3,867,632 and 4,013,887 to Wade L. Fite teach a less complicated and still effective way of accomplishing the dc ramp effect. This is to pass the ions through a hollow tube placed immediately before the quadrupole structure to which both rf and dc voltages are applied, said tube protruding slightly into the space between the four quadrupole rods and being made of a leaky dielectric material which appears as essentially a dielectric to rf fields and as a conductor to dc fields. The rf fringe fields penetrate the walls of the tube and appear within its interior; the dc fringe fields terminate on the outer surface of the tube and are not present within the tube. The incoming ions passing along the length of the hollow tube therefore "sees" the rf fields first and remain on stable trajectories while within the tube. On emergence from the tube, the ions "see" the dc fringe fields but because of the rf fields are already of substantial value, the ions remain on stable trajectories as they pass through the dc fringe fields.
U.S. Pat. No. 4,234,791 to C. G. Enke et al teaches the technique of triple quadrupole tandem mass spectrometry. The disclosures of this patent and others mentioned above are incorporated herein by reference. In the technique disclosed in the Enke et al patent, a first quadrupole mass filter structure is placed so that it is received ions from an ion source. The first quadrupole structure has both rf and dc voltage applied to it so that it selects and transmits ions of a given mass-to-charge ratio (parent ions). "Parent" ions emerging from the first quadrupole structure enter a collision cell into which an atomic or molecular gas has been admitted so that the parent ions collide with the collision gas atoms or molecules. This fragments parent ions and produces "daughter" ions. Within this collision cell is placed a second quadrupole structure, but one to which only rf voltages are applied. Because a quadrupole mass filter structure to which only rf voltages are applied transmits all ions with a mass-to-charge ratio greater than a value determined by the amplitude of the rf voltage of a given frequency, both parent ions and daughter ions with mass-to-charge ratios greater than the value determined by the amplitude of the rf voltage applied to the second quardupole structure emerge from the collision cell. These ions enter a third quadrupole mass filter to which both dc and rf fields are applied, where the mass-to-charge ratios of both parent and daughter ions are determined. The purpose of having the rf-only quadrupole structure inside the collision cell is to negate the effects of angular deflections in the fragmenting collisions. In the absence of the rf-only quardupole structure, daughter ions would for the most part be scattered to the walls of the collision cell and not emerge from the collision cell. By providing the rf-only quardupole structure, the fragment ions are confined to trajectories about the axis of the structure which ensures a vast majority of the daughter ions proceed along that axis, emerge from the collision cell and enter the third quadrupole mass filter. The amplitude of the rf voltage applied to the second quadrupole structure is normally less than applied to either of the first or third structure order to ensure that all daughter ions of interest are transmitted through the second quadrupole structure.
In most embodiments of the triple quadrupole tandem mass spectrometer, the collision cell is cylindrical and is constructed of metal. The end plate of the cell are also constructed of metal and have apertures at their centers to allow ions to enter and exit the collision cell. Gas leaks out of the collision cell through these apertures in sufficient quantities to require high pumping speed on the vacuum chamber housing the entire unit.
Because the end plates are constructed of electrically conducting metal, the rf and dc fringe fields between the first quadrupole and the entrance end plate of the collision cell are coincident in space, so that the ions emerging from the first quadrupole mass filter are on unstable trajectories as they approach the collision cell entrance aperture. Many of the ions that should enter the collision cell are therefore transversely rejected in the fringe fields and fail to enter the collision cell. A similar situation occurs at the exit end of the collision cell where the fringe fields between the exit end plate and the third quadrupole cause further rejection of the ions. These two ion rejection processes cause reduction of transmitted ions and therefore a loss of signal strength.
A common method to reduce the rejection of ions in the regions outside the two end plates is to place an electrostatic lens (also of metal) near both end plates, with its potential being sufficiently high that the ions are accelerated to higher velocities while passing through these regions of instability. By thus shortening the time that the ions are in regions of instability, some improvement of transmission is achieved.
A second common method to improve ion transmission is to make the apertures in the collision cell end plates large, so that some of the ions on unstable trajectories may be taken into the collision cell before they get too far away from the axis. But this approach increases the gas load to be evacuated by the pumps and is therefore undesirable.